Very low-sulfur fuel oil and method for producing the same

ABSTRACT

The present invention relates to a method for producing very low-sulfur fuel oil having high compatibility and high stability, comprising: mixing petroleum residua obtained from at least two different petroleum refining processes, adding a hydrocarbon solvent to the residual petroleum mixture, heating the mixture of the petroleum residua mixture and hydrocarbon solvent to extract and recover a mixture of oil fractions and the hydrocarbon solvent from the mixture of the petroleum residua mixture and hydrocarbon solvent with raffinate having asphaltenes therein being left, and removing the hydrocarbon solvent from the mixture of the oil fractions and the hydrocarbon solvent, thereby obtaining very low-sulfur fuel oil, wherein the very low-sulfur fuel oil has a sulfur content of 0.5 wt % or less bared on the total weight of the very low-sulfur fuel oil, and very low-sulfur fuel oil produced by the production method.

TECHNICAL FIELD

The present invention relates to a method for producing a verylow-sulfur oil having a sulfur content of 0.5 wt % or less, comprisingsteps of mixing petroleum residua obtained from at least two differentpetroleum refining processes for production of relatively high qualityfuel oils like gasoline, heavy oil, jet aircraft fuel, etc. to obtain apetroleum residua mixture; adding a hydrocarbon solvent to the petroleumresidua mixture to obtain a mixture of the petroleum residua mixturewith asphaltenes eliminated and the hydrocarbon solvent; heating mixtureof the petroleum residua mixture and the hydrocarbon solvent to extractand recover a mixture of oil fractions from the petroleum residuamixture and the hydrocarbon solvent with raffinate having asphaltenestherein being left, and recovering the hydrocarbon solvent from extractstream: and a very log sulfur fuel oil, particularly marine fuel oil,produced by the above method.

BACKGROUND ART

As International Marine Organization (IMO) regulation on the sulfurcontent of marine fuel oil will come into effect from 2020, the oilrefining industry has entered a new phase of marine fuel oil production.As the regulation standard on the sulfur content of marine fuel oilgreatly strengthened from the existing sulfur content of 3.5 wt % to asulfur content of 0.5 wt %, additional desulfurization treatment ofexisting marine fuel oil products has become essential.

The relevant literature presents low asphaltene content and higharomatics content as recommendations for ensuring the stability ofmarine fuel oil. It has been known that incompatibility of oil blendincreases due to aggregation of asphaltenes therein, when oils producedby different processes (e.g., straight-run fuel oil and cracked oil) oroils having greatly different viscosities and densities (e.g., FCC(Fluidized Catalytic Cracker) slurry oil and diesel oil) are mixedtogether to balance physical properties. In the case of an oilcontaining large amounts of saturated hydrocarbon compounds andasphaltenes, asphaltenes are not completely dispersed in the form ofmicelles therein due to relatively small amounts of aromatic compoundsthereof and thus, aggregagtion and precipitation of asphaltenes occur astime goes by. The oil produced by a single process is in generalregarded to have low stability, as described above. Even in the case ofa certain oil having a sufficient stability on its own, the tendency toform impurities such as sludge and precipitate may be accelerated in oilblend, resulting in insufficient incompatibility thereof, when it isblended with the other oil derived from other refining processes.

As the regulations on sulfur content have been greatly strengthened asintroduced above, it has become difficult to produce products with pricecompetitiveness using oil produced in a single process, such as existingmarine fuel oil, and it can be considered that most products areproduced by mixing very low-sulfur oil with existing high-sulfur oil. Inaddition, as the margin of oil refinery complex continues to be lowered,many oil refiners are tend to increase the input of low-quality crudeoil, which is high in acidity and contains excessive amounts ofimpurities such as sulfur, nitrogen, and metals, but has ion prices. Asthis tendency is intensified, the quality of marine fuel oil usingresiduum oil as a raw material may be degraded. Accordingly, theproportion of very low-sulfur oil such as ultra-low sulfur diesel thathave been blended in little amounts in high sulfur fuel oil has to beincreased rapidly to satisfy a sulfur content specification, and in somecases, it can be attempted to mix different residuum oils produced bytwo or more upgrading or cracking processes. These attempts can have asignificant defect on the stability of the oil as mentioned above.

Many analysis methods have been proposed as methods for measuring thestability and compatibility of oils. Among these methods, one of methodsthat enable eidetic determination is Spot Test (ASTM D4740). The ratingfor the Spot Test is shown in FIG. 1 . The rating for the Spot Test isreferred to as Spot Rating and can be rated from 1 to 5.

In the case of an oil containing large amounts of saturate andasphaltenes, even if it is not mixed with other oil, the spot rating mayworsen as the contents of solids and sludge increase due to theaggregation of asphaltenes over time. In the case of oil blend obtainedby mixing oils originated from different upgrading or crackingprocesses, the spot rating changes can be observed more immediately thanin the case of single oils. An oil showing spot rating of 3 or higher isvery unstable, and thus when it is stored in a marine oil tanker for along period of time, additional deterioration in the stability of theoil is inevitable, and in this case, normal operation of the purifierand engine in the marine fuel system is severely affected. Theproduction of stable marine fuel oils using residuum oil as a rawmaterial can be achieved through an additional limited process (vacuumresiduum desulfurization (VRDS)) using a production facility, which isvery advanced and requires excessive operating costs (due to the use ofexcessive hydrogen). However, this production process is not profitabledue to huge CAPEX, OPEX, and restricted types of feed oil. In thesituation where a huge amount of high-sulfur fuel oil, which has beenused on all international waters, has to be replaced by very low-sulfurfuel oil, if this very low-sulfur fuel oil is produced only through thelimited process, it will difficult to sufficiently meet the demand forthe very low-sulfur fuel oil, and the very low-sulfur fuel oil producedthrough this production will be disadvantageous in terms of pricecompetitiveness.

DISCLOSURE OF INVENTION Technical Problem

The present invention has been made in order to of the above-describedstability and compatibility problems in prior arts and disadvantagesoccurring when a plurality of different oil-fractions are mixedtogether, and it is an object of the present invention to provide amethod for producing very to fuel oil (VLSFO), which uses a relativelysimple production process, does not require much investment in theprocess, may use, as a raw material, oil-fractions from various upstreamprocesses, particularly petroleum residua remaining after production ofrelatively high quality fuel oil fractions and may also drasticallyreduce the production cost. That is, an object of the present inventionis to provide VLSFO having high stability (spot rating of 1) using, as araw material, petroleum residua produced in oil refining processes,which may be used for the production of marine fuel oil, and a mixtureof petroleum residua mixed at a predetermined ratio.

Solution to Problem

The present invention provides a method for producing very low-sulfurfuel oil, comprising steps of:

mixing petroleum residua obtained from at least two different petroleumrefining processes for production of relatively high quality fuel oilsto obtain a petroleum residua mixture;

adding a hydrocarbon solvent to the petroleum residua mixture to obtaina mixture of the petroleum residua mixture with asphaltenes eliminatedand the hydrocarbon solvent;

heating the mixture of the petroleum residua mixture and hydrocarbonsolvent to extract and recover a mixture of oil fractions from thepetroleum residua mixture and the hydrocarbon solvent with raffinatehaving asphaltenes therein being left; and

recovering the hydrocarbon solvent from the mixture of the oil fractionsand the hydrocarbon solvent, thereby obtaining very low-sulfur fuel oil,

wherein the very low-sulfur fuel oil has a sulfur content of 0.5 wt % orless based on the total weight of the very low-sulfur fuel oil.

The petroleum residua may be selected from the group consisting ofatmospheric residuum (AR), vacuum residuum (VR), hydrotreatedatmospheric residuum (t-AR), hydrotreated vacuum residuum (t-VR),deasphalted oil (DAO), hydrotreated deasphalted oil (t-DAO), unconvertedoil (UCO) (or HCR process residuum), vacuum gas oil (VGO), t-VGO(hydrotreated VGO), high-sulfur diesel (HSD), and ultra-low-sulfurdiesel (ULSD).

The hydrocarbon solvent may preferably be selected C₃-C₅ hydrocarbonsolvent or a mixture of two or more thereof. The C₃-C₅ hydrocarbonsolvent may preferably be selected from the group consisting ofn-propane, n-butane, i-butane, n-pentane, i-pentane, and a mixture oftwo or more thereof, and may more preferably be selected from the groupconsisting of n-pentane, i-pentane, and a mixture thereof. Mostpreferably, the C₃-C₅ hydrocarbon solvent may be n-pentane.

The hydrocarbon solvent is added with the volume ratio of 1 to 4:1 thehydrocarbon solvent to the petroleum residua mixture in an extractioncolumn at a pressure ranging from 30 to 50 barg (gauge pressure) and atemperature ranging from 100 to 230° C.

The mixture of the petroleum residua mixture and hydrocarbon solvent isheated to extract and recover a mixture of oil fractions and thehydrocarbon solvent with raffinate having asphaltenes therein beingleft.

A very low-sulfur fuel oil according to the present invention may beobtained by mixing petroleum residua obtained from at least twodifferent petroleum refining processes for production of relatively highquality fuel oils to obtain a petroleum residua mixture; adding C₃-C₅hydrocarbon solvent to the petroleum residua mixture to obtain a mixtureof the petroleum residua mixture and the hydrocarbon solvent, heatingthe mixture of the petroleum residua mixture and the hydrocarbon solventto extract and recover a mixture of oil fractions and the hydrocarbonsolvent with raffinate having asphaltenes therein being left; andrecovering the hydrocarbon solvent from the extract stream. The verylow-sulfur fuel oil of the present invention may have a sulfur contentof 0.5 wt % or less based on the total weight of the very low-sulfurfuel oil, and an asphaltene content of 0.1 to 0.6 wt %, more preferably0.05 to 0.55 wt %, most preferably 0.01 to 0.50 wt %, based on the totalweight of the very low-sulfur fuel oil.

Advantageous Effects of Invention

According to the present invention, very low-sulfur fuel oil having highcompatibility and high stability may be produced using, as a rawmaterial, a mixture of petroleum residua obtained from at least twodifferent petroleum refining processes as a substance remaining afterproduction of relatively high quality fuel oil fractions, and the mixingratio between the petroleum residua may be selected flexibly inconsideration of the operating status of petroleum refining processes.In addition, the production cost may be significantly reduced comparedto that of upgrading process such as hydro-desulfurization process,which use limited feedstocks and require high operating costs.

As demonstration of such effects, the very low-sulfur fuel oil (VLSFO)produced by the present invention shows a difference in behavior andstability from conventional marine fuel oil when it is introduced into apurifier and engine in a ship. This difference is analyzed to be due tothe change of components (mainly saturates, aromatics, resins andasphaltenes) that are inevitably contained in oil fractions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is photographs showing the spot rating of a spot test (ASTMD4740) for evaluating the stability of fuel oil.

FIG. 2 is a flowchart showing a production process of very lowsulfur-fuel oil by separating and removing asphaltenes from a petroleumresidua mixture according to the present invention.

MODE FOR THE INVENTION

A method of very low-sulfur fuel oil according to the present inventioncomprises steps of:

mixing petroleum residua obtained from at least two different petroleumrefining processes for production of relatively high quality fuel oilsto obtain a petroleum residua mixture;

adding a hydrocarbon solvent to the petroleum residua mixture to obtaina mixture of the petroleum residua mixture and the hydrocarbon solvent;

heating the mixture of the petroleum residua mixture and the hydrocarbonsolvent to extract and recover a mixture of oil fractions and thehydrocarbon solvent with raffinate having asphaltenes therein beingleft; and

recovering the hydrocarbon solvent from the mixture of the oil fractionsand the hydrocarbon solvent, thereby obtaining very low-sulfur fuel oil,wherein the very low-sulfur fuel oil has a sulfur content of 0.5 wt % orless based on the total weight of the very low-sulfur fuel oil.

The petroleum residua may be selected from the group consisting ofatmospheric residuum (AR), vacuum residuum (VR), hydrotreatedatmospheric residuum (t-AR), hydrotreated vacuum residuum (t-VR),deasphalted oil (DM)), hydrotreated deasphalted oil (t-DAO), unconvertedoil (UCO) (or HCR process residuum), vacuum gas oil (VGO), hydrotreatedvacuum gas oil(t-VGO) high-sulfur diesel (HSD), and ultra-low-sulfurdiesel (ULSD).

In the present specification, the term “high-quality oils” refers tooils such as jet aircraft oil and gasoline, which have low boilingpoints and high economic values, and the expression “petroleum residuum”as used in the present invention refers to an oil fraction, which isobtained from a petroleum refining process for production of relativelyhigh quality fuel oil fractions, mainly in the form of residuum, andhave a high sulfur and asphaltene content.

Specifically, according to the present invention, very low-sulfur fueloil may be produced by adding a C₃-C₅ hydrocarbon solvent to a mixtureobtained by mixing different kinds of petroleum residua at apredetermined ratio and separating asphaltenes, which is a sourcematerial that causes aggregation and precipitation, from the mixture ofpetroleum residua The C₃-C₅ hydrocarbon solvent may preferably beselected from the group consisting of n-propane, n-butane, i-butane,n-pentane, i-pentane, and a mixture of two or more thereof, may morepreferably be selected from the group consisting of n-pentane,i-pentane, and a mixture thereof, most preferably n-pentane.

For separation of asphaltenes in an extraction column, the ratio of theC₃-C₅ hydrocarbon solvent to the petroleum residua mixture is 1 to 4:1,more preferably 2 to 3:1, the pressure that is used for the separationis 30 to 50 barg, more preferably 35 to barg, most preferably 38 to 43barg, and the temperature that is used for the separation may beT_(c)(critical temperature of the hydrocarbon solvent) minus 20° C. toT_(c)plus 20° C., more preferably T_(c)−5° C. to T_(c)+15° C., mostpreferably T_(c)−10° C. to T_(c)+1.0° C. The temperature may range from100° C. to 230″C.

After separation of the asphaltenes, the hydrocarbon solvent is removedfrom the extract stream, thereby obtaining very low-sulfur fuel oil. Therecovered hydrocarbon solvent may be reused, and a stream of raffinatemay be used as a blending stock for conventional coker unit.

Through several repeated experiments, the present inventors have foundthat hydrocarbon solvents exhibit a stronger solvent effect as thenumber of carbon atoms in the hydrocarbon solvents increases, but when ahydrocarbon solvent having, a high solvent effect, such as hexane having6 carbon atoms, is used, the efficiency of removal of asphaltenes fromthe petroleum residua mixture is greatly reduced, and in the case ofhydrocarbon solvents having the same carbon number, a linear hydrocarbon(e.g., n-pentane) exhibits a stronger solvent effect than a branchedhydrocarbon (e.g., i-pentane). In addition, the present inventors havefound that the yield of VLSFO increases at low temperature and highpressure depending on the density change and thermodynamic preference ofthe supercritical hydrocarbon solvent in the extraction column in whichthe extraction of oil fractions is performed. The present inventors havefound that a proper hydrocarbon solvent needs to be used to maximize theyield of VLSFO while preventing the loss of oil fractions, and inparticular, have found that it is necessary to select a suitablehydrocarbon solvent to increase the stability of the mixture of oilfractions.

The very low-sulfur fuel of the present invention may have a sulfurcontent of 0.001 to 0.5 wt %, preferably 0.05 to 0.49 wt %, mostpreferably 0.1 to 0.48 wt %, based on the total weight of the verylow-sulfur fuel.

The very low-sulfur fuel according to the present invention producedfrom the mixture of petroleum residua exhibits improved storagestability.

That is, the petroleum residuum have a high content of saturates and/ora high content of asphaltene, and hence when this petroleum residuum areused as a raw material to produce fuel oil, the stability of the fueloil is low. However, according to the present invention, when two ormore petroleum residua selected from among atmospheric residuum (AR),vacuum residuum (VR), hydrotreated atmospheric residuum (t-AR),hydrotreated vacuum residuum (t-VR), deasphalted oil (DAO), hydrotreateddeasphalted oil (t-DAO), unconverted oil (UCO) (or HCR processresiduum), vacuum gas oil (VGO), t-VGO (hydrotreated VGO (vacuum gasoil), high-sulfur diesel (HSD), and ultra-low-sulfur diesel (ULSD) aremixed together at a predetermined ratio and the mixture is treated witha hydrocarbon solvent to remove asphaltenes, very low-sulfur fuel oilhaving high stability and meets specifications on the very low-sulfurfuel oil.

Hereinafter, the present invention will be described in detail withreference to examples.

EXAMPLE 1

In an extraction column, 1,457,000 liter of petroleum residua mixture,obtained by mixing t-AR and t-DAO at a volume ratio of 1:1, was mixedwith n-pentane solvent at a solvent/petroleum residua mixture volumeratio of 2 under conditions of 42 barg and 205° C., and extracted for 60minutes (extraction column residence time). Total extraction processoperation time was 660 minutes. Asphaltenes were removed therefrom sothat it remained in raffinate. The mixture of oil fractions from thepetroleum residua mixture and the solvent was extracted and recovered,and then the solvent was separated from the recovered mixture of oilfractions and solvent, thereby obtaining 1,394,000 liter of fuel oil.The obtained fuel oil was measured for its sulfur content, asphaltenecontent, spot rating by a spot test immediately after asphalteneremoval, and spot rating by a spot test during storage, and the resultsof the measurement are shown in Table 1 below. The sulfur content wasmeasured in accordance with ASTM D4294, and the asphaltene content wasmeasured in accordance with ASTM D6560.

EXAMPLE 2

In an extraction column, 1,643,000 liter of petroleum residua mixture,obtained by mixing t-AR and t-DAO at a volume ratio of 1:1, was mixedwith n-pentane solvent at a solvent/petroleum residua mixture volumeratio of 2 under conditions of 42 barg and 185° C., and extracted for 40minutes (extraction column residence time). Total extraction processoperation time was 480 minutes. Asphaltenes were removed therefrom sothat it remained in raffinate. The mixture of oil fractions from thepetroleum residua mixture and the solvent was extracted and recovered,and then the solvent was separated from the recovered mixture of oilfractions and solvent, thereby obtaining 1,615,000 liter of fuel oil.The obtained fuel oil was measured for its sulfur content, asphaltenecontent, spot rating by a spot test immediately after asphalteneremoval, and spot rating by a spot test during storage, and the resultsof the measurement are shown in Table 1 below.

EXAMPLE 3

In an extraction column, 1,325,000 liter of petroleum residua mixture,obtained by mixing t-AR and t-DAO at a volume ratio of 1:1, was mixedwith i-pentane solvent at a solvent/petroleum residua mixture volumeratio of 1 under conditions of 42 barg and 220° C., and extracted for 90minutes (extraction column residence time). Total extraction processoperation tune was 600 minutes. Asphaltenes were removed therefrom sothat it remained in raffinate. The mixture of oil fractions from thepetroleum residua mixture and the solvent was extracted and recovered,and then the solvent was separated from the recovered mixture of oilfractions and solvent, thereby obtaining 958,000 liter of fuel oil. Theobtained fuel oil was measured for its sulfur content, asphaltenecontent, spot rating by a spot test immediately after asphalteneremoval, and spot rating by a spot test during storage, and the resultsof the measurement are shown in Table 1 below.

EXAMPLE 4

In an extraction column, 1,656,000 liter of petroleum residua mixture,obtained by mixing t-AR and t-DAO at a volume ratio of 1:1, was mixedwith an n-pentane solvent at a solvent/petroleum residua mixture volumeratio of 1 under conditions of 42 barg and 220° C., and extracted for 70minutes (extraction column residence time). Total extraction processoperation time was 600 minutes. Asphaltenes were removed therefrom sothat it remained in raffinate. The mixture of oil fractions from thepetroleum residua mixture and the solvent was extracted and recovered,and then the solvent was separated from the recovered mixture of oilfractions and solvent, thereby obtaining 1,495,000 liter of fuel oil.The obtained fuel oil was measured for its sulfur content, asphaltenecontent, spot rating by a spot test immediately after asphalteneremoval, and spot rating by a spot test during storage, and the resultsof the measurement are shown in Table 1 below.

Comparative Example 1

In an extraction column, 1,325,000 liter of t-AR as single petroleumresiduum instead of petroleum residua mixture was mixed with ann-pentane solvent at a solvent/petroleum residuum volume ratio of 3under conditions of 42 barg and 190° C., and extracted for 70 minutes(extraction column residence time). Total extraction process operationtime was 600 minutes. Asphaltenes were removed therefrom so that itremained in raffinate. The mixture of oil fraction from the petroleumresiduum and the solvent was extracted and recovered, and then thesolvent was separated from the recovered mixture of the oil fraction andthe solvent, thereby obtaining 1,176,000 liter of fuel oil. The obtainedfuel oil was measured for its sulfur content, asphaltene content, spotrating by a spot test immediately after asphaltene removal, and spotrating by a spot test during storage, and the results of the measurementare shown in Table 2 below.

Comparative Example 2

Low-sulfur AR without removing asphaltenes was measured for its sulfurcontent, asphaltene content and spot rating by a spot test, and theresults of the measurement are shown in Table 2 below. Here, thelow-sulfur AR was an unhydrotreated atmospheric residuum having lowsulfur content.

Comparative Example 3

A mixture of petroleum residua, obtained by mixing low-sulfur AR andULSD at a volume ratio of 91: 9, was measured for its sulfur content,asphaltene content, spot rating by a spot test immediately afterasphaltene removal, and spot rating by a spot test during storage, in astate in which asphaltene was not removed therefrom. The results of themeasurement are shown in Table 2 below.

Comparative Example 4

A mixture of petroleum residua, obtained by mixing low-sulfur AR andULSD at a volume ratio of 76:24, was measured for its sulfur content,asphaltene content, spot rating by a spot test immediately afterasphaltene removal, and spot rating by a spot test during storage, in astate in which asphaltene was not removed therefrom. The results of themeasurement are shown in Table 2 below.

Comparative Example 5

A mixture of petroleum residua, obtained by mixing AR, ULSD and UCO1 ata volume ratio of 91:4.5:4.5, was measured for its sulfur content,asphaltene content, spot rating by a spot test immediately afterasphaltene removal, and spot rating by a spot test during storage, in astate in which asphaltene was not removed therefrom. The results of themeasurement are shown in Table 3 below.

Comparative Example 6

A mixture of petroleum residua, obtained by mixing AR, ULSD and SLO at avolume ratio of 72:20:8, was measured for its sulfur content, asphaltenecontent, spot rating by a spot test immediately after asphalteneremoval, and spot rating by a spot test during storage, in a state inwhich asphaltene was not removed therefrom. The results of themeasurement are shown in Table 3 below. Here, the SLO was residuum in afluidized catalytic cracking (FCC) process.

Comparative Example 7

A mixture of petroleum residua, obtained by mixing t-AR, LCO, SLO andH-Aro at a volume ratio of 80:9:6:5 was measured for its sulfur content,asphaltene content, spot rating by spot test immediately afterasphaltene removal, and spot rating by a spot test during storage, in astate in which asphaltene was not removed therefrom. The results of themeasurement are shown in Table 3 below. Here, the LCO was a kind oflow-price oil fraction produced in a fluidized catalytic crackingprocess, and the H-Aro was a by-product in an aromatic productionprocess.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Petroleum residuum t-ARt-DAO t-AR t-DAO t-AR t-DAO t-AR t-DAO Petroleum residuum S % (wt %)0.65 0.27 0.65 0.25 0.7 0.23 0.72 0.2 As % (wt %) 3.21 0.1 3.12 0.1 3.080.05 3.25 0.06 S.R. (1-5) 4 1 4 1 4 1 4 1 Mixing ratio (v/v) 50 50 50 5050 50 50 50 Petroleum residua S % (wt %) 0.48 0.46 0.47 0.47 mixture(before As % (wt %) 1.51 1.56 1.39 1.53 treatment with solvent) S.R.(1-5) 3   3   4   3   Solvent/petroleum residuum (v/v) 2   2   1   1  Solvent n-pentane n-pentane i-pentane n-pentane Process conditions(barg/° C.) 42/205 42/185 42/220 42/220 Oil fraction (after S % (wt %)0.48 0.47 0.46 0.46 treatment with solvent) As % (wt %) 0.24 0.60 0.160.57 S.R. (1-5) 1   1   1   1   Storage stability (days) 30+   30+  30+   30+   Yield (%) 96    98    72    90   

TABLE 2 Comp. Comp. Comp. Comp. Example 1 Example 2 Example 3 Example 4Petroleum residuum t-AR LSAR LSAR USLD LSAR USLD Petroleum residuum S %(wt %) 0.67 0.48 0.53 <0.001 0.58 <0.001 As % (wt %) 4.03 0.51 0.53<0.01 0.51 <0.01 S.R. (1-5) 4   1 1 1 1 1 Mixing ratio (v/v) — — 91 9 7624 Petroleum residua mixture S % (wt %) — — 0.48 0.45 (before treatmentAs % (wt %) — — 0.48 0.4 with solvent) S.R. (1-5) — — 1 2Solvent/petroleum residuum (v/v) 3   — — — Solvent n-pentane — — —Process conditions (barg/° C.) 42/190 Oil fraction (after S % (wt %)0.65 with solvent) As % (wt %) 0.26 S.R. (1-5) 1   Storage stability(days) 30+   14-18 14-18 14-18 Yield (%) 89    — — —

TABLE 3 Comp. Example 5 Comp. Example 6 Comp. Example 7 Petroleumresiduum LSAR USLD UCO LSAR USLD LSO t-AR LCO SLO H-Aro Petroleumresiduum S % (wt %) 0.55 <0.001 <0.001 0.56 <0.001 1.15 0.45 0.67 1.12<0.001 As % (wt %) 0.55 <0.01 <0.01 0.52 <0.01 3.52 3.48 <0.01 3.45<0.01 S.R. (1-5) 1 1 1 1 1 1 4 1 1 1 Mixing ratio (v/v) 91 4.5 4.5 72 208 80 9 6 5 Petroleum residua S % (wt %) 0.49 0.5 0.48 mixture (before As% (wt %) 0.51 0.68 3.04 treatment with solvent) S.R. (1-5) 2 3 4Solvent/petroleum residuum (v/v) — — — Solvent — — — Process conditions(barg/° C.) — — — Oil fraction (after S % (wt %) — — — treatment withsolvent) As % (wt %) — — — S.R. (1-5) — — — Storage stability (days)14-18 i i Yield (%) — — —

In Tables 1 to 3 above, S % denotes sulfur content, As % denotesasphaltene content, and S.R. denotes spot rating. In Table 3 above, “i”denotes immediately after mixing, and indicates that storage stabilitywas not determined due to high spot rating immediately after mixing.

Based on the results shown in Tables 1 to 3 above, the effects of thepresent invention will be described hereinbelow.

1. It could be confirmed that the Examples, in which asphaltene wasremoved from the petroleum residua mixture, all exhibited high stabilityafter asphaltene removal from low stability before asphaltene removal,suggesting that these Examples demonstrate the stability of the oilfraction according to the present invention. In addition, it could beconfirmed that, in the storage stability of the oil fraction, due toremoval of asphaltene, the Examples showed a spot rating of 1 withouttime-dependent changes even after 30 days, suggesting that the storagestability of the oil fraction was also greatly improved.

2. It could be confirmed that Example 1, in which asphaltene was removedfrom the mixture of two petroleum residua, exhibited the stability andstorage stability comparable to Comparative Example 1, in whichasphaltene was removed from a single petroleum residuum. In addition, itcould be confirmed that the yield of Example 1 was higher than that ofComparative Example 1 with a reduced solvent amount and elevated processtemperature compared to Comparative Example 1.

3. When comparing Examples 1 and 2, in which asphaltene was removed fromthe same petroleum residua mixture and the same solvent was used in thesame amount, it could be confirmed that the yield was improved bylowering the process temperature. This suggests that the yield may varydepending on the density of the supercritical solvent in the extractioncolumn.

4. Then comparing Examples 3 and 4, in which asphaltene was removed frompetroleum residua mixture and the solvent was used in the same amount,it could be confirmed that the yield of Example 4, in which n-pentane isused, was greatly improved compared to that of Example 3 in whichi-pentane was used. This also suggests that the yield may vary dependingon presence of branching in the solvent and the density of thesupercritical solvent in the extraction column.

5. Comparative Examples 2 and 3, in which asphaltene was not removed,showed a spot rating of 1 immediately after mixing, and thus exhibitedstability, but the spot rating was degraded to 2 after 14 to 18 days ofstorage after mixing.

6. Comparative Examples 4 and 5, in which asphaltene was not removed,showed a spot rating of 2 immediately after mixing, but the spot ratingwas degraded to 3 alter 14 to 18 days of storage after mixing. Thus, itcould be confirmed that the Comparative Examples (Comparative Examples 2to 5), which showed high stability immediately after mixing, showeddeterioration in storage stability.

7. The petroleum residua mixtures of Examples 6 and 7, from whichasphaltene was not removed and which included the petroleum residuumhaving a high asphaltene content, showed low stability even immediatelyafter mixing.

Although only the petroleum residua mixture comprising t-AR and t-DAOwas described in the above Examples, similar results could also beobtained for other mixture of petroleum residua as listed above,remaining after producing high-quality oils.

While the present invention has been described above with reference tothe specific embodiments, it is to be understood that variousmodifications are possible without departing front the scope of thepresent invention. Therefore, the scope of the present invention shouldnot be limited by the described embodiments, but should be defined notonly by the appended claims, but also the equivalents of the claims.

1. A method for producing very low-sulfur fuel oil, comprising steps of:mixing petroleum residua obtained from at least two different petroleumrefining processes for production of relatively high quality fuel oilsto obtain a petroleum residua mixture; adding a hydrocarbon solvent tothe petroleum residua mixture to obtain a mixture of the petroleumresidua mixture and the hydrocarbon solvent; heating of the mixture ofthe residual petroleum mixture and hydrocarbon solvent to extract andrecover a mixture of oil fractions from the petroleum residua mixtureand the hydrocarbon solvent with raffinate having asphaltenes thereinbeing left; and removing the hydrocarbon solvent from the mixture of theoil fractions and the hydrocarbon solvent, thereby obtaining verylow-sulfur fuel oil; wherein the very low-sulfur fuel oil has a sulfurcontent of 0.5 wt % or less based on the total weight of the verylow-sulfur fuel oil.
 2. The method of claim 1, wherein the petroleumresidua are selected from the group consisting of atmospheric residuum(AR), vacuum residuum (VR), hydrotreated atmospheric residuum (t-AR),hydrotreated vacuum residuum (t-VR), deasphalted oil (DAO), hydrotreateddeasphalted oil (t-DAO), unconverted oil (UCO) (or HCR processresiduum), vacuum gas oil (VGO), t-VGO (hydrotreated VGO vacuum gasoil), high-sulfur diesel (HSD), and ultra-low-sulfur diesel (ULSD). 3.The method of claim 1, wherein the hydrocarbon solvent is C₃-C₅hydrocarbon solvent or a mixture of two or more thereof.
 4. The methodof claim 3, wherein the C₃-C₅ hydrocarbon solvent is selected from thegroup consisting of n-propane, n-butane, i-butane, n-pentane, andi-pentane.
 5. The method of claim 1, wherein the hydrocarbon solvent isadded with the volume ratio of 1 to 4:1 of the hydrocarbon solvent tothe petroleum residua mixture in an extraction column at a pressureranging from 30 to 50 barg and a temperature ranging from 100° C. to230° C.
 6. A very low-sulfur fuel oil which is obtained by a method forproducing very low-sulfur fuel oil, comprising steps of: mixingpetroleum residua obtained from at least two different petroleumrefining processes for production of relatively high quality fuel oilsto obtain a petroleum residua mixture; adding a hydrocarbon solvent tothe petroleum residua mixture to obtain a mixture of the petroleumresidua mixture and the hydrocarbon solvent; heating of the mixture ofthe residual petroleum mixture and hydrocarbon solvent to extract andrecover a mixture of oil fractions from the petroleum residua mixtureand the hydrocarbon solvent with raffinate having asphaltenes thereinbeing left; and removing the hydrocarbon solvent from the mixture of theoil fractions and the hydrocarbon solvent, thereby obtaining verylow-sulfur fuel oil, wherein the very low-sulfur fuel oil has a sulfurcontent of 0.5 wt % or less based on the total weight of the verylow-sulfur fuel oil.
 7. The very low-sulfur fuel oil of claim 6, whereinthe petroleum residua are selected from the group consisting of AR, VR,t-AR, t-VR, DAO, t-DAO, UCO, VGO, t-VGO, HSD, and ULSD.
 8. The verylow-sulfur fuel oil of claim 6, having an asphaltene content of 0.01 to0.6 wt % based on the total weight of the very low-sulfur fuel oil. 9.The very low-sulfur fuel oil of claim 6, wherein the hydrocarbon solventis C₃-C₅ hydrocarbon solvent or a mixture of two or more thereof.